Prototype and commercial dual-subtype FIV vaccines, consisting of inactivated subtype-A and -D strains, conferred sterilizing protection against homologous subtype, subtype-A/B recombinant, and heterologous subtype-B challenges. The mechanisms of dual-subtype FIV vaccine protection in cats should provide insights to the development of effective HIV-1/AIDS vaccine in humans. Passive antibody immunization with purified dual-subtype FIV vaccine-induced immunoglobulin to na[unreadable]ve cats afforded protection of the recipient cats against homologous subtype, which correlated with the presence of vaccine-induced virus neutralizing antibodies (VNA). In contrast, passive protection was not achieved against heterologous subtype-B challenge with VNA-resistant strain. Moreover, adoptive transfer (A-T) of T-cell enriched population from vaccinated cats to MHC-matched cats protected the A-T recipient against homologous and heterologous challenges. Dual-subtype vaccination using combined immunization routes (subcutaneous, intradermal, transcutaneous, & intranasal) afforded protection against homologous vaginal challenge. These results from the previous funding cycle suggest that protection against vaccine-induced VNA-susceptible strains (homologous subtype strains) is mediated by both VNA immunity and cell-mediated immunity (CMI), while vaccine protection against heterologous subtype strains is mediated by CMI. The studies in the competitive renewal grant are aimed at identifying the phenotypes and functions of the T cells as well as the viral epitopes and MHC profiles responsible for the dual-subtype vaccine protection (Specific aim 1). The proposed studies will also identify the best vaccination route(s) and the mechanisms of vaccine protection against mucosal-vaginal challenges with heterologous strains from homologous or heterologous subtype (Specific aim 2). The ultimate goal of these studies is to provide insights about which viral components, host immune responses, MHC profiles, and vaccination routes are important for an effective prophylaxis against the predominant transmission modes (mucosal and intravenous) of HIV-1 in humans.